1. Field of the Invention
The present invention relates to a communication apparatus, a communication terminal and a program recording medium, which are used for constituting a local area network.
2. Description of the Related Art
Over the recent years, the information has been exchanged by use of a local area network (hereinafter referred to as a LAN) in a variety of fields. A construction and a fundamental operation of the LAN used at the present will be described by exemplifying a bus type LAN with reference to FIG. 10.
As illustrated in FIG. 10, the LAN is constructed of a transmission line, a plurality of stations 31 and a plurality of nodes 34 (known as network interface cards). The stations 31 are defined as apparatuses (e.g., computers) actually employed by users who exchange the information by using the LAN. The node 34 is defined as a communication apparatus for connecting the station 31 to the transmission line, and is constructed of a controller 32 and a transceiver 33. It is to be noted that a specific construction (configuration) of the node 34 is different depending on the specifications of the LAN and the construction of the station 31. For example, in the LAN based on the specifications classified as 10BASE2, if the station 31 is a disk top type computer, a board type apparatus insertable into an extension slot of the computer is used as a node 34. By contrast, in the LAN based on the specifications classified as 10BASE5, the board inserted into the extension slot is an apparatus corresponding to the controller 32, and the transceiver 33 is connected to this board (the controller 32) via a cable.
The node 34 executes a process of converting data outputted by the station 31 into a signal for the transmission line, and a process reverse thereto. More specifically, the controller 32 within the node 34, when indicated by the station 31 to transmit the data, creates a packet by adding, to this item of data, pieces of information such as a transmitting address and a receiving address (which are layer-2 addresses called a LAN address or a MAC address), and transfers the thus created packet to the transceiver 33. The transceiver 33 converts a form of the signal of the transferred packet and transmits it onto the transmission line.
When the packet is transmitted via the transmission line, the transceiver 33 receives this packet, converts the signal form and transfers it to the controller 32. The controller 32, if the receiving address within the transferred packet is coincident with an address allocated to the controller 32 itself (hereinafter referred to as a self-address), transfers the intra-packet data to the station 31. Note that the self address is normally printed inwardly of the controller 21 by the manufacturer. Whereas if the intra-packet receiving address is not coincident with the self-address, the controller 32 disposes of the transferred packet without transferring the intra-packet data to the station 31.
For instance, when the data transmitted from a station C to a station A, as shown in FIG. 10, a packet in which "A" is set in the receiving address and "C" is set in the transmitting address, is sent to the transmission line from a node C. Both of the node A and the node B receive this packet, however, the node B discards the received packet because of the receiving address not being the self-address. While on the other hand, the node A transfers the data contained in that packet to the station A because of the receiving address being the self-address. In the LAN, the data transmitted from the station C to the station A thus arrives at only the station A.
In the normal LAN, a protocols explained so far is used in combination with a high-layer protocol (TCP/IP etc.) incorporating a communication processing function to some extent. In the high-layer protocol, an address (a layer-3 address called a network address etc) different from the LAN address is used. Therefore, the packet transmitted onto the transmission line is in fact the one holding the data containing the address used in the high-layer protocol, and any one of the controller 32 and the station 31 implements control (a process relative to the high-layer protocol) making use of the network address.
As explained above, the prior art LAN is designed for freely exchanging the information between the stations. Hence, it was difficult to perform a test etc by use of the conventional LAN. Further, when an important conference using the LAN takes place, it might happen that data not related to the conference are received by the stations engaged in the conference, which can hinder the conference may occur.